Chapter 14 Nuclear Magnetic Resonance Spectroscopy. Copyright 2011 The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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1 Chapter 14 Nuclear Magnetic Resonance Spectroscopy Copyright 2011 The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1

2 Nuclear Magnetic Resonance Spectroscopy Nuclear magnetic resonance spectroscopy is a powerful analytical technique used to characterize organic molecules by identifying carbon-hydrogen frameworks within molecules. Two common types of NMR spectroscopy are used to characterize organic structure: 1 H NMR is used to determine the type and number of H atoms in a molecule; and 13 C NMR is used to determine the type of carbon atoms in a molecule. The source of energy in NMR is radio waves which have long wavelengths, and thus low energy and frequency. When low-energy radio waves interact with a molecule, they can change the nuclear spins of some elements, including 1 H and 13 C. 2

3 Magnetic Fields in NMR When a charged particle such as a proton spins on its axis, it creates a magnetic field, causing the nucleus to act like a tiny bar magnet. Normally, these tiny bar magnets are randomly oriented in space. However, in the presence of a magnetic field (B 0 ), they are oriented with or against this applied field. More nuclei are oriented with the applied field because this arrangement is lower in energy. The energy difference between these two states is very small (<0.1 cal). 3

4 Energy and Nuclear Spin In a magnetic field, there are now two energy states for a proton: a lower energy state with the nucleus aligned in the same direction as B 0, and a higher energy state in which the nucleus aligned against B 0. When an external energy source (hν) that matches the energy difference (ΔE) between these two states is applied, energy is absorbed, causing the nucleus to spin flip from one orientation to another. The energy difference between these two nuclear spin states corresponds to the low frequency RF region of the electromagnetic spectrum. 4

5 Resonance Energy Two variables characterize NMR: An applied magnetic field B 0, the strength of which is measured in tesla (T) The frequency ν of radiation used for resonance, measured in hertz (Hz), or megahertz (MHz) A nucleus is in resonance when it absorbs RF radiation and spin flips to a higher energy state. 5

6 Resonance Frequency The frequency needed for resonance and the applied magnetic field strength are proportionally related: The stronger the magnetic field, the larger the energy difference between the two nuclear spin states, and the higher the ν needed for resonance. NMR spectrometers are referred to as 300 MHz instruments, 500 MHz instruments, and so forth, depending on the frequency of the RF radiation used for resonance. These spectrometers use very powerful magnets to create a small but measurable energy difference between two possible spin states. 6

7 NMR Schematic Figure

8 The frequency at which a particular proton absorbs is determined by its electronic environment. Protons in different environments absorb at slightly different frequencies, so they are distinguishable by NMR. The size of the magnetic field generated by the electrons around a proton determines where it absorbs. Only nuclei that contain odd mass numbers (such as 1 H, 13 C, 19 F, and 31 P) or odd atomic numbers (such as 2 H and 14 N) give rise to NMR signals. Electron Environment 8

9 1 H NMR Spectra An NMR spectrum is a plot of the intensity of a peak against its chemical shift, measured in parts per million (ppm). 9

10 Interpreting NMR Spectra NMR absorptions generally appear as sharp peaks. Most protons absorb between 0-10 ppm. The terms upfield and downfield describe the relative location of peaks. Upfield means to the right (higher magnetic field). Downfield means to the left (lower magnetic field). An external standard, (CH 3 ) 4 Si (tetremethylsilane) is added to the sample tube prior to the NMR measurement. NMR absorptions are measured relative to the position of a reference peak at 0 ppm on the δ scale due to tetramethylsilane (TMS). TMS is a volatile inert compound that gives a single peak upfield from typical NMR absorptions. 10

11 Chemical Shift The chemical shift of the x axis gives the position of an NMR signal, measured in ppm, according to the following equation: By reporting the NMR absorption as a fraction of the NMR operating frequency, we get units, ppm, that are independent of the spectrometer. 11

12 Structural Information from Features of a 1 H NMR Spectrum Number of signals: indicates the number of different types of hydrogen in a molecule. Position of signals: indicates what types of hydrogen the molecule contains. Intensity of signals: indicates the relative amounts (how many) of each kind of hydrogen in the molecule. Spin-spin splitting of signals: gives further information of the neighboring environment for the various hydrogens in the molecule. 12

13 Number of Signals in 1 H NMR The number of NMR signals equals the number of different types of protons in a compound. Protons in different environments give different NMR signals. Equivalent protons give the same NMR signal. 13

14 Number of Signals: Equivalent Hydrogens Equivalent hydrogens: have the same chemical environment; Look for symmetry in the molecule a molecule with 1 set of equivalent hydrogens gives 1 NMR signal O CH 3 CCH 3 ClCH 2 CH 2 Cl H 3 C C C CH 3 H 3 C CH 3 Propanone (Acetone) Cyclopentane 1,2-Dichloroethane 2,3-Dimethyl- 2-butene TMS 13-14

15 Equivalent Hydrogens Number of 1 H- NMR signals Cl CH 3 CHCl O Cl C H CH 3 C H 1,1-Dichloro- Cyclopent- (Z)-1-Chloro- Cyclohexene Number of 13 C- NMR signals OCH 3 OCH OCH O 3 OCH OCH

16 Number of 1 H NMR Signals Examples Figure 14.2 Another way to determine if two hydrogens in a molecule are the same: replace each hydrogen by a different atom, Z (e.g., Cl) and determine if the resulting compound is the same or different. For example, replacement of each methyl hydrogen at C 1 and C 5 in pentane produces 1-chloropentane, replacement of each methylene hydrogen at C 2 and C 4 leads to 2-chloropentane and at C 3 forms 3-chloropentane. 16

17 Number of 1 H NMR Signals Alkenes In comparing two H atoms on a ring or double bond, two protons are equivalent only if they are cis (or trans) to the same groups. 17

18 Number of 1 H NMR Signals Cycloalkanes To determine equivalent protons in cycloalkanes and alkenes, always draw all bonds to hydrogen and determine whether or not they are cis (or trans) to the same groups. 18

19 Shielding of Nuclei In the vicinity of the nucleus, the magnetic field generated by the circulating electron opposes the applied field and decreases the external magnetic field that the proton feels. Since the electron experiences a lower magnetic field strength, it needs a lower frequency to achieve resonance. Lower frequency is to the right in an NMR spectrum, toward a lower chemical shift, so shielding shifts the absorption upfield. 19

20 Shielding and Resonant Frequency The less shielded the nucleus becomes, the more of the applied magnetic field (B 0 ) it feels. This deshielded nucleus experiences a higher magnetic field strength, so it needs a higher frequency to achieve resonance. Higher frequency is to the left in an NMR spectrum, toward higher chemical shift so deshielding shifts an absorption downfield. Protons near electronegative atoms are deshielded, so they absorb downfield. 20

21 Shielding and Chemical Shift Figure

22 Summary of Shielding Figure

23 Shielding and Signal Position 23

24 Characteristic Chemical Shifts Protons in a given environment absorb in a predictable region in an NMR spectrum. 24

25 Substitution and Chemical Shift The chemical shift of a C H bond increases with increasing alkyl substitution. 25

26 Aromatic Deshielding In a magnetic field, the six π electrons in benzene circulate around the ring creating a ring current. The magnetic field induced by these moving electrons reinforces the applied magnetic field in the vicinity of the protons. The protons thus feel a stronger magnetic field and a higher frequency is needed for resonance meaning they are deshielded and absorb downfield. 26

27 Alkene Chemical Shifts In a magnetic field, the loosely held π electrons of the double bond create a magnetic field that reinforces the applied field in the vicinity of the protons. The protons now feel a stronger magnetic field, and require a higher frequency for resonance meaning the protons are deshielded and absorb downfield. 27

28 Alkyne Chemical Shifts In a magnetic field, the π electrons of a carbon-carbon triple bond are induced to circulate, but in this case the induced magnetic field opposes the applied magnetic field (B 0 ). The proton thus feels a weaker magnetic field, so a lower frequency is needed for resonance. The nucleus is shielded and the absorption is upfield. 28

29 Summary of π Electron and Chemical Shift 29

30 Regions in the 1 H NMR Spectrum Figure

31 Intensity of 1 H NMR Signals The area under an NMR signal is proportional to the number of absorbing protons. An NMR spectrometer automatically integrates the area under the peaks, and prints out a stepped curve (integral) on the spectrum. The height of each step is proportional to the area under the peak, which in turn is proportional to the number of absorbing protons. 31

32 1 H NMR Integration Modern NMR spectrometers automatically calculate and plot the value of each integral in arbitrary units. The ratio of integrals to one another gives the ratio of absorbing protons in a spectrum. Note that this gives a ratio, and not the absolute number, of absorbing protons. 32

33 33

34 1 H NMR Spin-Spin Splitting The spectra up to this point have been limited to single absorptions called singlets. Often signals for different protons are split into more than one peak. 34

35 1 H NMR Spin-Spin Splitting Spin-spin splitting occurs only between nonequivalent protons on the same carbon or adjacent carbons. How does the doublet due to the CH 2 group on BrCH 2 CHBr 2 occur? When placed in an applied electric field, (B 0 ), the adjacent proton (CHBr 2 ) can be aligned with ( ) or against ( ) B 0. Thus, the absorbing CH 2 protons feel two slightly different magnetic fields one slightly larger than B 0, and one slightly smaller than B 0. They absorb at two different frequencies in the NMR spectrum, thus splitting a single absorption into a doublet. 35

36 Coupling Constants The frequency difference, measured in Hz between two peaks of the doublet is called the coupling constant, J. 36

37 How Triplets Arise When placed in an applied magnetic field (B 0 ), the adjacent protons H a and H b can each be aligned with ( ) or against ( ) B 0. Thus, the absorbing proton feels three slightly different magnetic fields: one slightly larger than B 0 one slightly smaller than B 0 one the same strength as B 0 Because the absorbing proton feels three different magnetic fields, it absorbs at three different frequencies in the NMR spectrum, splitting a single absorption into a triplet. 37

38 Triplets Because there are two different ways to align one proton with B 0, and one proton against B 0 that is, a b and a b the middle peak of the triplet is twice as intense as the two outer peaks, making the ratio of the areas under the three peaks 1:2:1. When two protons split each other, they are said to be coupled. The spacing between peaks in a split NMR signal, measured by the J value, is equal for coupled protons. 38

39 Formation of Triplets 39

40 Splitting Patterns Three general rules describe the splitting patterns commonly seen in the 1 H NMR spectra of organic compounds. [1] Equivalent protons do not split each other s signals. [2] A set of n nonequivalent protons splits the signal of a nearby proton into n + 1 peaks. [3] Splitting is observed for nonequivalent protons on the same carbon or adjacent carbons. 40

41 Proximity and Splitting Splitting is not generally observed between protons separated by more than three σ bonds. 41

42 42 42

43 1 H NMR of 2-Bromopropane Figure 14.6 The 6 H a protons are split by the one H b proton to give a doublet. The H b proton is split by 6 equivalent H a protons to yield a septet. 43

44 More Complex Splitting Patterns When two sets of adjacent protons are different from each other (n protons on one adjacent carbon and m protons on the other), the number of peaks in an NMR signal = (n + 1)(m + 1). Figure

45 1 H NMR of 1-Bromopropane Since H a and H c are not equivalent to each other, we cannot always add them together and use the n + 1 rule. However, since the coupling constants, J ab and J bc, are very similar, the signal for H b is a sextet (follows the n + 1 rule). Figure

46 Coupling Constants for Alkenes Protons on carbon-carbon double bonds often give characteristic splitting patterns. A disubstituted double bond can have two geminal protons, two cis protons, or two trans protons. When these protons are different, each proton splits the NMR signal of the other so that each proton appears as a doublet. The magnitude of the coupling constant J for these doublets depends on the arrangement of hydrogen atoms. 46

47 Splitting Patterns for Alkenes Figure H NMR spectra for the alkenyl protons of (E)- and (Z)-3-chloropropenoic acid 47

48 Splitting Diagram for Vinyl Acetate Splitting diagrams for the alkenyl protons in vinyl acetate are shown below. Each pattern is different in appearance because the magnitude of the coupling constants forming them is different. Figure Splitting diagram for the alkenyl protons in vinyl acetate (CH 2 =CHOCHOCH 3 ) 48

49 1 H NMR of Vinyl Acetate Figure

50 Protons on Benzene Rings Benzene has six equivalent deshielded protons and exhibits a single peak in its 1 H NMR spectrum at 7.27 ppm. Monosubstituted benzenes contain five deshielded protons that are no longer equivalent, and the appearance of these signals is highly variable, depending on the identity of Z. Figure

51 51

52 1 H NMR Structure Determination, continued 52

53 1 H NMR Structure Determination, continued 53

54 1 H NMR Structure Determination, continued 54

55 13 C NMR Spectrum Example 13 C Spectra are easier to analyze than 1 H spectra because the signals are not split. Each type of carbon atom appears as a single peak. 55

56 Splitting in 13 C NMR The lack of splitting in a 13 C spectrum is a consequence of the low natural abundance of 13 C. Splitting occurs when two NMR active nuclei like two protons are close to each other. Because of the low natural abundance of 13 C nuclei (1.1%), the chance of two 13 C nuclei being bonded to each other is very small (0.01%), and so no carbon-carbon splitting is observed. A 13 C NMR signal can also be split by nearby protons. This 1 H- 13 C splitting is usually eliminated from the spectrum by using an instrumental technique that decouples the proton-carbon interactions, so that every peak in a 13 C NMR spectrum appears as a singlet. 56

57 Details of 13 C NMR The two features of a 13 C NMR spectrum that provide the most structural information are the number of signals observed and the chemical shifts of those signals. The number of signals in a 13 C spectrum gives the number of different types of carbon atoms in a molecule. Because 13 C NMR signals are not split, the number of signals equals the number of lines in the 13 C spectrum. In contrast to the 1 H NMR situation, peak intensity is not proportional to the number of absorbing carbons, so 13 C NMR signals are not integrated. 57

58 Chemical Shifts in 13 C NMR In contrast to the small range of chemical shifts in 1 H NMR (1-10 ppm usually), 13 C NMR absorptions occur over a much broader range (0-220 ppm). The chemical shifts of carbon atoms in 13 C NMR depend on the same effects as the chemical shifts of protons in 1 H NMR. 58

59 13 C NMR of 1-Propanol Figure

60 13 C NMR of Methyl Acetate Figure cont d 60

61 Magnetic Resonance Imaging (MRI) Figure

62 How to determine structures of compounds from their spectra? 1. Determine the molecular formula and degree of unsatura5on 2. Determine the func5onal group present from IR 3. From 13 C NMR, determine # of signals and chemical shibs 4. From 1 H NMR, determine # of signals, # of protons in each signal, peak splifng pagerns and chemical shibs

63 Molecular formula: C4H7O2Cl, IR absorp5on peak at 1740 cm 1 Cl O O

64 Molecular formula: C5H10O O

65 Molecular formula: C7H7Br Br

66 Formula C4H8O O

67 Formula C10H12O2 O

68 Formula C5H10O2 O O

69

70 Formula C10H9NO2 NC O O

71 Formula C11H14O O

72 Cyclohexane Conformers Recall that cyclohexane conformers interconvert by ring flipping. Because the ring flipping is very rapid at room temperature, an NMR spectrum records an average of all conformers that interconvert. Even though each cyclohexane carbon has two different types of hydrogens one axial and one equatorial the two chair forms of cyclohexane rapidly interconvert them, and an NMR spectrum shows a single signal for the average environment that it sees. 72